1. Field of the Invention
The invention relates generally to robotic substrate handling systems. More particularly, the invention relates to an apparatus and a method for on-the-fly substrate orientation and center finding while a substrate is transferred between chambers by a robot handling system.
2. Background of the Related Art
In the manufacture of integrated circuits, semiconductor substrates are loaded into various reaction and other processing chambers using automated equipment for processing. Typically, the automated equipment includes a robot that can transfer a semiconductor substrate, such as a silicon wafer, from a cassette in a loadlock chamber through a central transfer chamber and into one or more processing chambers disposed in connection to the transfer chamber. The robot is typically disposed in a central location in the transfer chamber to provide access to all of the chambers connected to the transfer chamber. It is desirable to know the exact location and orientation of the semiconductor substrate relative to the processing chamber so that the substrate can be precisely positioned at an optimum location within the processing chamber to maximize the effectiveness of the processing onto the precise desired surface area of the substrate to be processed. Preferably, the center point and the notch or flat orientation indicator on the substrate are determined to position the substrate in alignment with the desired processing position. Likewise, it is also desirable that the substrate positioning apparatus which is used as a reference point and upon which the substrate is transported be routinely calibrated so that positioning error is minimized, if not eliminated.
One current method and system for locating the center point and the notch or flat orientation indicator of semiconductor substrates provides a "spindle" type orientation/center-finding apparatus. A substrate is transferred by a shuttle robot to a spindle in the orientation/center-finding apparatus where the substrate is incrementally rotated to determine the center and orientation of the substrate. The distances between the center of rotation to the periphery of the wafer is measured along a linear path by a sensor means, the wafer centerpoint offset is calculated by geometric analysis of the measurements, and the wafer centered on the spindle by the shuttle robot.
There are several disadvantages with the spindle type method and system. First, it is an entirely separate and distinct apparatus from the processing system. Having a separate center-finding apparatus requires an additional step in the manufacturing process, adding cost and complexity and decreasing valuable throughput time. That is, the wafer cannot be directly unloaded by robot from the wafer storage cassette and transferred to a processing chamber without first being manipulated by the separate center-finding apparatus. As a result, the spindle type system and method does not take advantage of the direct movement of the wafer as it is transferred from the wafer storage cassette to the processing chamber. In addition, the shuttle robot may require periodic calibration by a separate calibration tool if the center-finding method is to remain accurate. Furthermore, once the positioning method has been performed, the wafer is transferred to a separate wafer transport arm which may also require periodic calibration to maintain precision positioning of the wafer.
Another system for locating the centerpoint of semiconductor substrates is disclosed in Cheng et al., U.S. Pat. No. 4,819,167, entitled SYSTEM AND METHOD FOR DETECTING THE CENTER OF AN INTEGRATED CIRCUIT WAFER, issued Apr. 4, 1989, which patent is hereby incorporated by reference in its entirety and is commonly assigned to Applied Materials, Inc., of Santa Clara, Calif., the Assignee of the present invention.
In Cheng et al., the system and method disclosed is of an "optical sensor array" type whereby a semiconductor wafer is moved along a linear path across an array of sensors positioned generally transverse to the linear path of the wafer support blade. This centerfinder method is performed upon the direct removal of the wafer from a storage cassette by a processing system robot and while en route to a processing chamber. The robot blade and peripheral edges of the wafer are detected separately by the optical sensors to calculate the coordinate center position of the wafer relative to the robot blade. An x-y coordinate system is defined by the path (x) of movement of the robot arm/blade and the center line (y) of the optical sensors. The origin (0) of the y coordinate axis is defined by the position of the center sensor. The detection of the robot blade provides a reference point and origin (0,0) of the x-y coordinate system from which to move the wafer to its destination point. The detection of points along the leading and trailing edges of the wafer provide points on the x axis generally parallel to the path of movement of the wafer and from which the centerpoint of the wafer can be geometrically determined. Once the wafer center position is geometrically determined, the wafer can be moved and positioned at the destination location.
The Cheng et al. type centerfinding system overcomes the disadvantages of having a separate and distinct apparatus. The centerpoint of the wafer is determined directly during movement of the wafer to its destination location. This is especially advantageous in a wafer processing system configuration where there exists a robot of a R-Theta type in a multiple chamber processing apparatus with a single loadlock chamber as shown in Cheng et al. However, the Cheng et al type centerfinding system does not provide orientation of the substrate to align the notch or flat orientation indicator on the substrate to a particular orientation. The substrate has to be provided in an oriented position in the load lock cassette or has to be oriented in a separate orientation apparatus.
Therefore, there is a need for a combined orientation/centerfinding system that provides on-the-fly substrate orientation and center finding features as the substrate is transferred between chambers connected to a central transfer chamber.